1. Field of the Invention
The invention relates to a fuel supply system of an internal combustion engine and, more particularly, to a fuel supply system equipped with a fuel property sensor for determining the property of the fuel that is used by the internal combustion engine.
2. Description of Related Art
Some of the internal combustion engines that are in practical use today are able to use plural kinds of fuels that are different in property. However, the property of the fuel to be used is not always known, and is not necessarily constant. For example, in the case of alcohol blended gasoline, there are plural kinds of alcohol blended gasoline that differ in alcohol concentration. Therefore, it sometimes happens that, at the time of refilling a fuel tank, a fuel whose alcohol concentration is different from that of the fuel in the fuel tank is fed. Therefore, an internal combustion engine that is expected to use a plurality of kinds of fuels having different properties needs to have means for detecting the property of the fuel that is being used.
The foregoing means to be employed may be a fuel property sensor. Japanese Patent Application Publication No. 2009-133273 (JP 2009-133273 A) discloses a fuel supply system equipped with an alcohol concentration sensor that is a type of fuel property sensor. In the system disclosed in this publication, a fuel flow path that connects a fuel pump and a fuel injector is provided with an alcohol concentration sensor. According to this system, it is possible to measure the alcohol concentration of the fuel that is supplied to the injector by the fuel pump, that is, of the fuel that is to be injected from the injector into the cylinder in a certain time.
By the way, the determination of the fuel property through the use of a fuel property sensor is sometimes performed for the purpose of detecting degradation of the fuel in the tank or introduction of an abnormal fuel into the tank. For that purpose, it is desirable that the fuel property be determined before the fuel is supplied to the internal combustion engine. However, in the case where the fuel flow path connecting a fuel pump and an injector is provided with a fuel property sensor as in the above-described system, determination of the property of an object fuel becomes possible only after the object fuel is supplied from the fuel pump to the injector. That is, the object fuel has to be actually used before the fuel property thereof can be determined. Besides, in the case where the vehicle in which the internal combustion engine is mounted is a hybrid vehicle (in particular, a plug-in hybrid vehicle), there are only a few opportunities at which the internal combustion engine is started and the fuel is consumed. Therefore, in the case where the fuel flow path connecting the fuel pump and the injector is provided with a fuel property sensor, there is a risk that detecting degradation of the fuel or the refueling with an abnormal fuel is impossible for a long period of time.